feat: advanced features — dashboard, budgets, history, export

Backend:
- GET /api/v1/dashboard?month=YYYY-MM: KPIs, by_category, 6-month trend, budget alerts
- GET/POST/PUT/DELETE /api/v1/budgets: budget envelopes with spent_cents/remaining_cents
- POST /api/v1/budgets/rollover: copy budgets from M-1 to target month
- GET /api/v1/history?year=YYYY: monthly summary for the year
- GET /api/v1/export/csv|pdf?month=YYYY-MM: StreamingResponse exports (WeasyPrint PDF)
- New schemas: dashboard, budget, history
- Services: dashboard_service, budget_service
- Routers mounted in main.py

Frontend:
- DashboardPage: 4 KPI cards, PieChart (expenses by category), BarChart (6-month trend),
  month navigation, budget alert badges, CSV/PDF export buttons
- BudgetsPage: progress bars (green/orange/red), create/edit form, delete, rollover M-1
- HistoryPage: annual table with month click → dashboard, LineChart revenues/expenses
- CategoriesPage: list by type with create/edit/delete (was missing from Phase 2)
- TransactionsPage: added CSV/PDF export buttons
- App.tsx: full routing with ProtectedRoute + Layout for all authenticated pages
- New hooks: useDashboard, useBudgets (with mutations), useHistory
- API types + client updated for all new endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/history.py

@@ -0,0 +1,14 @@
+from pydantic import BaseModel
+
+
+class MonthSummary(BaseModel):
+    month: str  # YYYY-MM
+    income_cents: int
+    expense_cents: int
+    balance_cents: int  # month net
+    transaction_count: int
+
+
+class HistoryResponse(BaseModel):
+    year: int
+    months: list[MonthSummary]